Research and development of various display devices have been carried out in image information systems according to a variety of demands such as high resolution, wide screen, thin type and low power consumption. Display devices using Braun tubes have mainly been used up to now, but there is a limit to making it thinner. Recently, research and development of FEDs are actively being carried out to achieve a thin type and low power consumption display according to such demands.
An FED has a structure, wherein a flat-shaped field-emission electron source is placed at the back plane of an enclosed vacuum box and a phosphor layer at the inside surface of a front faceplate, and by irradiating the phosphor layer with a low accelerating voltage electron beam on the order of approximately 0.1-10 kV accelerating voltage, the phosphor glows and an image is displayed. Herein, the electron density of the electron beam irradiated onto the phosphor layer is about 10-1000 times higher than that of a conventional Braun tube, thereby low resistance characteristics are required in the FED phosphor layer so as not to cause electron charge saturation Moreover, excellent characteristic of life under high electron density and high luminance characteristics with low luminescence saturation are also necessary.
Heretofore, a variety of research has been carried out to realize a long-life phosphor layer. Regarding crystal defects of the phosphor, as written in nonpatent document 1, crystal defects of a ZnS:Ag,Al phosphor were analyzed by observation with a transmission electron microscope (TEM), in phosphor No. P55 it is reported that the luminescent maintenance factor is excellent because of a small number of crystal defects. Additionally, as reported in Non-patent document 2, by irradiating an electron beam onto the ZnS phosphor, sulfur on the surface is eliminated, and formation of ZnO progresses. As a method to improve the performance of a phosphor for an FED, there is a means, as described in Patent document 1 for instance, to improve the crystallinity of a blue SrGa2S4:Ce phosphor layer by annealing in a reduced pressure atmosphere containing H2S. Moreover, although it is for electroluminescence (EL) and not for an FED, as described in nonpatent document 2 there is a means that an SrS system luminescence layer is treated in a sulfur atmosphere to reduce impurities and control the addition of luminescence centers.
Heretofore, various methods have been examined to achieve low resistance, long life, and high luminescence of phosphor layers for FEDs. However, all the problems have not been solved by the methods of the prior art. Especially, a new means is necessary to achieve long phosphor life.    [Patent document 1] JP-A 261367/1998    [Patent document 2] JP-A 192813/2001    [Non-patent document 1] J. Vac. Sci. Technol. A19 (2001) 1083    [Non-patent document 2] J. Vac. Sci. Technol. A14 (1996) 1697